It is known to pivotally mount a bulldozer blade or the like on the front end of the vehicle so that through selective manipulation of a pair of hydraulic tilt jacks connected to the blade at the opposite sides thereof its tilt and tip positions can be varied. At least one other lift jack is usually associated with the blade for changing its elevation. In order to permit easier and more responsive tilting or tipping of the blade, the hydraulic control system associated therewith should preferably have the capability to permit simultaneously coordinated operation of the tilt jacks for rapid tipping of the blade without affecting the tilt of the blade or simultaneously opposite coordinated movement thereof for tilting of the blade. A further desirable feature of the system is its flexibility to permit independent operation of the tilt jacks. For example, the bulldozer blade must frequently be rapidly positioned by manipulating the various levers while at the same time maneuvering the vehicle. This requires a high degree of dexterity and alertness on the part of the operator. Therefore, such control system should permit manually independent or simultaneous operation of the tilt jacks in order to minimize operational problems and to reduce operator fatigue.
Illustrative of the wide range of efforts to solve the many problems associated with varying the position of a bulldozer blade in U.S. Pat. No. 3,705,631 issued Dec. 12, 1972 to D. H. Seaberg. Unfortunately, while the fluid control system of the reference patent does allow simultaneous movement of the tilt jacks, it will not allow independent operation thereof. Because one jack feeds another, it is not possible to tilt the blade at a fully tipped condition. This deficiency results from one of the jacks being at the end of its stroke and adversely stopping flow to the other tilt jack. Another disadvantage of the referenced system is that it includes pilot operated check valves in the conduits leading to the tilt jacks to insure that the load on the blade will not cause the operationally affected tilt jack from extending or retracting faster than the system flow capability will allow. Such check valves add complexity and expense to the system.
Representative of another attempt to solve these problems is the control circuit shown in U.S. Pat. No. 3,774,696 issued Nov. 27, 1973 to R. Horsch. Such control circuit incorporates valving for connecting the tilt jacks in series or in parallel for respectively tilting or tipping the blade, but on the other hand lacks flow control so that the blade loading can adversely influence the desired positioning thereof. For example, when tipping the blade under load, there is no guarantee of equal fluid flow to both jacks and, consequently, the blade can tilt during tipping so that additional blade manipulation is required of the operator to reorient it.
Another general problem involves the compatability and integration of the tilt jack circuitry with the associated lift jack circuitry. Since the lift jacks require a considerably higher flow demand than the tilt jacks, it is desirable to provide a pair of pumps with different volumetric capacities therefor. Consequently, if a relatively large pump and associated system valving is used for directing flow to the lift jacks, and a smaller pump is used with appropriate valving for delivering flow to the tilt jacks, it is desirable for reasons of operating economy to utilize flow from the smaller pump and tilt jack circuit to supplement the flow of the larger pump in order to permit faster response of the lift jacks.